Martingale collar fits looser than a regular collar does. It can tighten to the point of choking your dog or making your dog uncomfortable, but this is okay as long as the collar is able to quickly slacken. On the other hand, the collar should not fit too loosely, as a loose fit defeats the purpose of the collar because your dog can slip out of it. If the two d-rings connecting the larger collar to the smaller loop meet one another when you pull on the d-ring connecting the smaller loop to the lead, then the collar fits too loosely. If you can fit your entire hand between the collar's larger loop and your dog's neck, the collar fits properly for regular wear. For training your dog not to pull on the lead, the collar should fit more snugly, and fitting two fingers between your dog's neck and the large loop indicates the right fit in this case. Please do not leave the dog at home alone wearing the collar.